This invention relates to a metal oxide varistor having unique electrical characteristics and methods for making the same.
Metal oxide varistors possess several electrical characteristics which make them useful as, for example, surge arrestors for protecting electrical equipment from transients on AC power lines created by lightning strikes or switching electrical apparatus. Sakshaug et al., in a paper entitled "Metal Oxide Arresters on Distribution Systems--Fundamental Considerations," presented at the IEEE/PES meeting of winter 1989, discusses the use of metal oxide varistors as surge arresters. Varistors can also be used for driving liquid crystal displays.
A metal oxide varistor, in general, exhibits a nonlinear current-voltage relationship which may be represented by the equation I=(V/C).sup..alpha., where V is the voltage between two points separated by the varistor material, I is the current flowing between the points, C is a constant, and .alpha. (or alpha) is a measure of the varistor nonlinearity. .alpha. can be calculated by the following equation: EQU .alpha.=log(I.sub.2 /I.sub.1)/log(V.sub.2 /V.sub.1)
where V.sub.1 and V.sub.2 are the voltages across the varistor at currents I.sub.1 and I.sub.2, respectively. Metal Oxide varistors generally comprise zinc oxide, together with minor amounts of other metal oxides, and typically have an alpha value of greater than 20. If the voltage applied to the varistor is less than a critical value (the varistor switching voltage) the varistor is essentially an insulator and only a small leakage current will flow through it. It is desirable that the leakage current through the varistor be as low as possible to prevent degradation of the varistor over time due to heat generated by the leakage current. If the applied voltage is greater than the switching voltage, the varistor resistance drops to low values permitting large currents to flow through it. At voltages above the switching voltage, the current through the varistor varies greatly for small changes in applied voltage so that the voltage across the varistor is effectively limited to a narrow range of values. The voltage limiting or clamping action is enhanced at higher values of alpha. It is also desirable that a varistor have high energy handling capability, that is the varistor should be capable of absorbing the maximum energy to which it is likely to be subjected. The energy absorption capability can be calculated as the product of the current, voltage and time when a square wave current is applied across the varistor for 2,000 microseconds. Other desirable electrical characteristics of a varistor are a low leakage current after the application of a high current impulse, a low AC watts loss temperature coefficient, and a small percent change in the AC watts loss after the application of a high current impulse.
Copending commonly assigned application No. 07/187,165, filed Apr. 28, 1988, of Thompson et al. discloses a varistor having among other characteristics high energy handling capabilities. Such a varistor is prepared from a precursor powder prepared as disclosed in copending commonly assigned application No. 07/193,970, filed May 13, 1988, also of Thompson et al. The disclosures of both applications are incorporated herein by reference.